Notes 249 



15. Nor is this the case x>nly with the eye: it is the same 

 with every other sense ; precise instances of this kind in regard 

 to the taste, the smell, the touch, etc., will occur plentifully to 

 every one. 



16. I consider this solution of the appearances of the col- 

 ours as perfectly satisfactory. Here it is applied only to one 

 instance, but it is equally applicable to all the rest ; and it 

 appears to me to account for all the difficulties which seem 

 to have embarrassed Count Rumford, in his very ingenious and 

 entertaining paper {Phil. Trans. 1794, p. 107). Also in Dr. 

 Priestley's History of Optics, p. 436, there is a curious chap- 

 ter, containing the observations of philosophers on blue and 

 green shadows ; the true cause of these shadows is not, I think, 

 there mentioned ; and it may be entertaining to read that 

 chapter with these principles in the mind. 



17. When the sun has been near setting on a summer even- 

 ing, I have often observed most beautiful blue shadows upon 

 a white marble chimney-piece. In this case, the weak white 

 light of the evening, which illumines the shaded part of the 

 marble, is to be considered as compounded of two colours, 

 orange and blue. The direct orange rays of the sun at this 

 time render the orange part invisible, and leave the blue in 

 perfection. 



18. And in the same way is to be explained that beautiful 

 and easy experiment mentioned by Count Rumford (p. 103, 

 Phil. Trans. 1794,) where a burning candle in the daytime 

 produces two shadows, and one of them of a most beautiful 

 blue colour. The experiment is the more valuable, as it may 

 be made at any time of the day with a burning candle. Almost 

 darken a room, and then by means of a lighted candle and 

 a little daylight produce two shadows of any small object, as of 



seeing a green colour j and the same reasoning holds in all other cases where the colour 

 are contrasts to each other. For such colours seem incapable of mixing with each other, 

 in the proper sense of the word, as when red and yellow are mixed together, and pro- 

 ducc a compound evidently partaking of the obvious properties of the two ingredients. 

 When contrasts are mixed together, as red and green, these colours seem destructive 

 of each other, and effect a compound approaching to whiteness. Similar observations 

 may be made on the other senses. 



